Trimming with Aluminum Coil Stock

by Martin Holladay
Aluminum coil stock, which has been used for years to cover
trim on re-siding jobs, is becoming standard on new homes, too
&Ntilde; part of the trend toward low-maintenance
exteriors.
Manufacturers of aluminum trim coil offer two types of
finish: smooth and striated. Smooth trim coil, which has been
on the market the longest, is a thin aluminum flashing finished
with polyester or acrylic paint. Striated coil has a textured
finish, providing a better visual match to vinyl siding.
Some manufacturers refer to striated coil as PVC-coated
coil. Jerry McKie, national sales and marketing manager at
First American Coil, notes that striated trim coil, like smooth
coil, is basically painted aluminum. "All it is, is paint,"
says McKie. "It's paint with vinyl chloride in it."
The PVC paint used for striated coil is thicker than the
paint used for smooth coil. The dry film thickness of the paint
on striated coil measures about 3.5 mils, compared to the 1-mil
paint film on smooth coil. Because the PVC coating is applied
so thickly, it forms striation patterns as it dries. "The
striations depend on the speed of the roller as well as the
properties of the paint," says Dan Hawk, metal products manager
at Alcoa. From a distance, the striations look a little bit
like wood grain.

Nominal vs. Actual
Thickness

In the U.S., the gauge of almost all aluminum trim coil sold,
whether smooth or striated, is nominal 0.019 inch. Many people
in the coil industry acknowledge that actual measurements of
aluminum coil products are often less than their nominal sizes.
"The coil is referred to as nominal .019 inch, but most of the
time it is thinner," says Mike Lohuis, product manager for
Gentek Building Products, a trim coil manufacturer. "There's a
plus or minus tolerance for aluminum trim coil. The .019 can be
as thin as .0175 before paint and still be within the
specifications. The aluminum industry is able to manufacture to
closer tolerances than some other industries, and because of
price pressures, manufacturers are making the material thinner
and thinner."
For installers who prefer a heavier coil, most manufacturers
also offer a heavy-duty trim coil with a nominal measurement of
0.022 or 0.024 inch, at an upcharge of about 15% to 20%.

Which Should You Use?

Nationwide, manufacturers sell about the same amount of
striated trim coil as smooth, although there are regional
differences in sales. Siding installers in the Northeast have
always preferred smooth coil, while buyers in the South prefer
striated.
Fans of smooth coil say that it resists marring from tools
better than striated coil. "The vinyl-coated coil marks up a
little easier, so you have to be careful with it," says John
Fiderio, a remodeler and siding installer from Meriden, Conn.
Mike Lohuis agrees. "The PVC-coated will take more abrasion
than the smooth coil, but it mars easier," he says.
Where aluminum trim is subject to degradation from air
pollution, the striated trim coil may be a better choice. "PVC
is known to be very chemical resistant, so if an application is
near an industrial site, or a highway, or acid rain, the PVC
coating will protect the surface better than the smooth," says
Dan Hawk from Alcoa.
A
third alternative to smooth or striated aluminum coil is an
all-vinyl bendable trim coil called Pro-Trim. Pro-Trim, which
has been on the market for nine years, is a true match for
vinyl siding. One of its advantages, compared to painted
aluminum, is its resistance to dents and scratches. However,
Pro-Trim has a much greater coefficient of thermal expansion
than aluminum has, so it needs to be installed with special
slotted fasteners and double-sided tape.

Most manufacturers of aluminum
trim coil offer a variety of colors in both smooth
and striated finishes.

Installers who are used to the sharp creases of aluminum
trim may not like the softer look of vinyl bends. Finally,
Pro-Trim is available in only five colors, far fewer than
aluminum coil. Pro-Trim has won converts among siding
installers offering all-vinyl exteriors, but it represents a
small fraction of the trim coil market.

Watch Out for PT Lumber

Many builders don't realize that aluminum trim coil is
incompatible with pressure-treated lumber. Aluminum flashing in
contact with pressure-treated lumber can disintegrate
completely in just two years. "The corrosion arises from the
copper in the pressure-treated lumber and generally occurs in
extremely wet locations," says Dan Hawk. "If you need to cover
a pressure-treated post, wrap the post first with tar paper or
housewrap and use stainless-steel nails to minimize the
potential for corrosion."

Although most aluminum trim coil
is used on re-siding jobs, its use in new
construction is growing. In the Northeast, most
siding installers prefer smooth aluminum trim coil
to striated trim coil.

Installation Details

If trim coil is fastened too tight, it will ripple when the
weather gets hot. Jobs with rippled trim have given aluminum
trim coil a bad reputation with some builders. But
conscientious installers can avoid the wavy look.
Rippling is caused by expansion, not contraction. "If you
install in hot weather, there is relatively little trouble,"
says Dan Parks, metal product manager at trim coil manufacturer
Napco Building Products, who spent years installing trim coil.
"But if you face-nail when the weather is cold, you'll get
rippling. The warmer the coil, the better. It helps to put the
coil in a heated garage or on the south side of the roof for a
while."
Other tricks for a quality trim coil installation:

Use a heavy-duty trim coil (nominal 0.022 or 0.024
inch).

Hem the material to stiffen it.

Drill or punch an oversized hole in the aluminum before
nailing.

Don't nail too snugly.

Use only aluminum or stainless-steel nails.

On rakes and fascias, install a 1x2 at the top of the
trim to provide a stepped profile. This stiffens the trim
and limits rippling.

Reduce face-nailing to a minimum. To avoid face-nailing
a fascia, insert the top of the trim coil under the
drip-edge or into a piece of undersill trim and install a 1
1/4-inch bend at the bottom toward the soffit. Nail up
through the bottom leg every 18 to 24 inches.